Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

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That slightly disturbing image (click to embiggen) is not a microscopic picture of a scientist's colon (at least, not as far as you know). It's actually a region near the Martian south pole. It was taken with the HiRISE camera on the Mars Reconnaissance Orbiter, and the area shown is roughly 700 meters across (about 0.4 miles).

What you're seeing are layers of the polar ice cap. The ice is mostly CO2. Mars is so cold that a lot of the cap persists throughout the year, and is called the residual cap. Some of it does sublimate, though, which means it goes right from a solid to a gas. Underneath that layer is something more solid, perhaps water ice, that does not sublimate. As the upper layer partially goes away, it leaves these weird Swiss-cheese-like patterns, revealing the smooth layer below.

This image only shows a small portion of a much larger area where this occurs. Here's the "context image", a zoom out if you will:

I marked the rough outline of the zoomed image in this one (it's rotated about 90 degrees counterclockwise in the zoom). You can see that this odd terrain (aresain?) goes on for kilometers. It really does look like some sort of bacterial colony. But it's actually the result of millennia, maybe millions of years, of constant annual atmospheric deposition and sublimation.

And just as a reminder -- because I love to point this out -- Mars was 250 million kilometers (150 million miles) away from Earth on August 20, 2009, when this image was obtained. Yet MRO was only 250 km above its target, yielding this fine imagery at a resolution of 25 centimeters (10 inches) per pixel. Got a ruler handy? Pick it up, hold it in your hand, and think on the fact that we have spacecraft orbiting Mars, an alien world, that can take pictures of objects on its surface about the size of that ruler.

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